The invention concerns a process for the preparation of fully desalted process water by treating the raw water with cation and anion exchangers in a separate multi-stage arrangement. In the following, process water signifies water, which is used both as water of addition for chemical, respectively physical processes, and as feed water for the production of steam. The collective concept process water is appropriate with the present-day coupling of steam-energy systems with process systems, both of the steam side and on the condensate side.
In the case of complete desalting systems, the degree of purity with respect to residual salts, acids and alkali is measured through the conductivity. A penetration of salts, acids or alkali through the desalting system has a devastating effect on the apparatus of the process system, in which the process water is used.
A large number of processes are known for the preparation of fully desalted process water; reference is made to the handbook "Wasser" (water), published by Vereinigte Kesselwerke AG, Duesseldorf, Issue of 1974, pages 101 to 135.
The state of the art is described, for example, by means of two characteristic processes, which are used for raw water with different analyses.
(a) Complete desalting process with strong acid cation exchanger, strong basic anion exchanger and mixed bed filter in accordance with FIG. 1a of the accompanying drawings.
The de-basification of the physically clean raw water is carried out in the cation exchanger a. By means of acid regeneration, the cation exchanger is charged with H.sup.+ ions and is able to exchange all cations of the water against H.sup.+ ions. This process is the de-basification because all cations are removed.
The removal of the anions of the weak and strong mineral acids, which are still contained in the water, is carried out in the anion exchanger b. This means that the anion exchanger is in a position to exchange the anions CL.sup.-, SO.sub.4.sup.2-, NO.sub.3, CO.sub.3.sup.2-, SiO.sub.3.sup.2- against the accumulated OH ions.
The improvement of the desalting and desilification effect takes place in the mixed bed exchanger c. The mixed bed exchanger contains a mixture of strong acid cation and strong basic anion exchange material, which, during the operation is present in an intimately mixed solid bed condition. The adjacent cation and anion resin particles represent a very long chain of series-arranged cation and anion exchangers. The good desalting effect of the mixed bed exchanger is based on this fact.
(b) Complete desalting process with weak acid cation exchanger, strong acid cation exchanger, weak basic anion exchanger, CO.sub.2 scrubbing tower, strong basic anion exchanger and a mixed bed filter in accordance with FIG. 1b of the accompanying drawing.
The decarbonation of the physically clean raw water is carried out in the weak acid cation exchanger d. The weak acid cation exchanger is charged with H.sup.+ ions by means of acid regeneration and can exchange the cations of the weak acids against the accumulated H.sup.+ ions.
The de-basification of the clean raw water is carried out in the strong acid cation exchanger e. By means of acid regeneration, the cation exchanger is charged with H.sup.+ ions and is able to exchange all cations of the water against H.sup.+ ions. This process is the de-basification, because all cations are removed.
The removal of the anions of the strong mineral acids is carried out in the weak basic anion exchanger f. This means that the weak basic anion exchanger is in a position to exchange the anions CL.sup.-, SO.sub.4.sup.2-, NO.sub.3.sup.- against the accumulated OH ions.
The expulsion of carbonic acid is carried out in the CO.sub.2 scrubbing tower g. The carbonic acid has been generated from the hydrogen carbonates in the cation exchanger. In the CO.sub.2 scrubbing tower, the water is vaporized in the upper portion and then runs over several levels, which are filled with plastic rings. In a counterflow, air is added to the water. By reducing the partial pressure for CO.sub.2, the carbonic acid is removed to a value of less than 10 mg/l.
The anions of the weak mineral acids are removed by means of the strong basic anion exchanger h. This means that the strong basic anion exchanger exchanges the weak mineral acids such as CO.sub.3.sup.2-, SiO.sub.3.sup.2- againt accumulated OH ions.
The improvement of the desalting and desilification effect takes place in the mixed bed exchanger j. The mixed bed exchanger contains a mixture of strong acid cation and strong basic anion exchange material, which, during the operation, is present in an intimately mixed solid bed condition. The adjacent cation and anion resin particles represent a very long chain of series-arranged cation and anion exchangers. The good effect of the residual desalting of the mixed bed exchanger is based on this fact.
FIGS. 1a and 1b are only examples, because a large number of variations are possible here. However, it is a significant characteristic that all configurations required a mixed bed filter so far.
The mixed bed filter following the anion exchangers becomes necessary because of the so-called sodium slip in the cation exchanger or exchangers. Because the cation exchangers cannot be designed for a residual content of Na compounds of practically zero, Na compounds leave the cation exchanger or exchangers in the order of magnitude of about 0.5 mg/l or more. These compounds are split in the strong basic anion exchanger into caustic soda in accordance with the equation NaCl+OH.d .fwdarw.Cl.d 3 NOH (1) and cause a high conductivity of the mixed bed exchanger. d=ion exchanger mass.
The sodium slip is in the ratio of the monovalent to the bivalent cations in the water, which is to be purified. The slippage can be generally reduced or avoided by means of the following additional measures:
1. Air flushing of the cation exchanger prior to and after the regeneration for the purpose of the distribution of the exchanger layer in the lower area of the filter, which is more heavily charged with Na.sup.+ ions.
2. The use of larger quantities of hydrochloric acid for regeneration.
3. The application of counterflow regeneration.
4. The introduction of an additional cation exchanger following the weak basic anion exchanger for the purpose of the residual splitting of the Na compounds coming out of the first cation exchanger. For FIG. 1b, this means the interposition of an additional cation exchanger after position 3.
The consumption of chemicals of a complete desalting system, designed in accordance with FIG. 1b, for 200 tons of fully desalted process water per hour, with a raw water analysis of:
______________________________________ pH value 7.50 Total hardness 17.2 .sup.o d Carbonate hardness 8.6 .sup.o d CaO 125 mg/l MgO 34 " CO.sub.2 - free 37 " NO.sub.3 46 " Cl 46 " SO.sub.4 109 " SiO.sub.2 9 " KM.sub.n O.sub.4 consumption 4 " is calculated as: HCl 30 percent 1,644 tons per year NaOH 50 percent 699 tons per year ______________________________________
If, on the basis of an analysis, which deviates from the above example, a greater sodium slip results, the consumption of chemicals and the required resources for apparatus and measuring technology would be considerably greater as a consequence of measures 1 to 4.